Physicalism (also known as Materialistic Monism - see the sections on Materialism and Monism) is the philosophical position that everything which exists is no more extensive than its physical properties, and that the only existing substance is physical. Therefore, it argues, the mind is a purely physical construct, and will eventually be explained entirely by physical theory, as it continues to evolve. With the huge strides in science in the 20th Century (especially in atomic theory, evolution, neuroscience and computer technology), Physicalism of various types (see below) has become the dominant doctrine in the Mind/Body argument (see the section on Philosophy of Mind).

The term "physicalism" was first coined by the Austrian philosopher Otto Neurath (1882 - 1945) in the early 20th Century. In some ways, the term "physicalism" is a preferable one to the closely related concept of Materialism because it has evolved with the physical sciences to incorporate far more sophisticated notions of physicality than just matter, for example wave/particle relationships and non-material forces produced by particles. Physicalism can also be considered a variety of Naturalism (the belief that nature is all exists, and that all things supernatural therefore do not exist).

An important concept within Physicalism is that of supervenience, which is the idea that higher levels of existence are dependent on lower levels, such that there can only be a change in the higher level if there is also a change in the lower level (the higher level is said to supervene on the lower level).

Objections to Physicalism point out the apparent contradiction of the existence of qualia (properties of sensory experiences, or "the way things seem to us") in an entirely physical world (also known as the knowledge argument). Hempel's Dilemma (propounded by the German philosopher Carl Hempel) attacks how Physicalism is defined: if, for instance, one defines Physicalism as the belief that the universe is composed of everything known by physics, one can point out that physics cannot describe how the mind functions; if Physicalism is defined as anything which may be described by physics in the future, then one is really saying nothing. Against this, it can be argued that many examples of previously dualistic concepts are being eroded by continuous scientific progress, and that the physical basis of the mind will almost certainly be known sometime in the future.

There are two main categories of Physicalism, Reductive and Non-Reductive:

Reductive Physicalism, which asserts that all mental states and properties will eventually be explained by scientific accounts of physiological processes and states, has been the most popular form during the 20th Century.
There are three main types:

Behaviourism, which holds that mental states are just descriptions of observable behaviour and that such behaviours can be described scientifically without recourse either to internal physiological events or to hypothetical constructs such as the mind.

Type Identity Theory (also known as Identity Theory or Type Physicalism), which holds that specific mental states are identical to specific physical internal states of the brain.

Functionalism, which holds that mental states (beliefs, desires, being in pain, etc.) are constituted solely by their functional role (the causal relations of mental states to other mental states, sensory inputs, and behavioural outputs), and can be characterized in terms of non-mental functional properties. It further asserts that mental states are multiply realizable, meaning that they can be sufficiently explained without taking into account the underlying physical medium (e.g. the brain, neurons, etc.) so that they can be realized in multiple ways, including, theoretically at least, within non-biological systems such as computers.

Non-Reductive Physicalism, which argues that, although the brain is all there is to the mind, the predicates and vocabulary used in mental descriptions and explanations cannot be reduced to the language and lower-level explanations of physical science. Thus, mental states supervene (depend) on physical states, and there can be no change in the mental without some change in the physical, but they are not reducible to them.
There are three main types:

Anomalous Monism, which states that mental events are identical with physical events, but that the mental is anomalous i.e. these mental events are perfectly real, and identical with (some) physical matter, but not regulated by strict physical laws. Therefore, all mental things are physical, but not all physical things are necessarily mental. This doctrine was first proposed by Donald Davidson in the 1970s.

Emergentism, which involves a layered view of nature, with the layers arranged in terms of increasing complexity, each corresponding to its own special science.

Eliminativism (or Eliminative Materialism), which holds that people's common-sense understanding of the mind ("folk psychology") is hopelessley flawed, and will eventually be replaced (eliminated) by an alternative, usually taken to be neuroscience.